1. Field of Invention
The present invention relates to a component in anti-theft installation. More particularly, the present invention relates to a metal base assembly inside a speaker capable of generating a high tone warning sound.
2. Description of Related Art
Due to the rapid economic development of our society, the material quality of our life continues to improve. Transport vehicles such as cars or motorcycles are often used to convey people to and from work or for leisure travel. In fact, they have become an almost indispensable part in the life of most families.
In general, most new cars or motorcycles are equipped with two anti-theft systems for burglary prevention. In addition to a lock guarding the engine starter, door locks and an anti-theft alarm system are frequently included for added security.
Among the many installations in an anti-theft system, the most important one is a high-tone speaker. The high tone warning sound produced by a speaker is quite effective in scaring off would-be burglars. The high tone sound of a speaker is generated by the rapid vibration of an acoustic diaphragm whose motion is in turn induced by a rapidly changing magnetic field.
The rapidly changing magnetic field is produced by supplying a current and forming a complete magnetic circuit through a metal base. FIG. 1 is an exploded view of components in a conventional anti-theft speaker together with an unit assembled from these components. A speaker 20 as shown in FIG. 1 comprises several components, including: a speaker casing 10, a mounting block 12, an acoustic diaphragm assembly 14, a plastic pad 16 and a metal base 18 for forming a magnetic circuit.
The speaker casing 10 has a plurality of guiding pins 22 protruding from the bottom surface. The guiding pins 22 are inserted into holes (not shown) on the mounting block 12 during an assembling process. The mounting block 12 also has a few holes (not shown) reserved for putting screws 30 in so that the acoustic diaphragm assembly 14, the plastic pad 16 and the metal base 18 can be attached together. The mounting block 12 also has a few slots permitting wires from the acoustic diaphragm assembly 14 and other structural elements to exit.
The acoustic diaphragm assembly 14 couples with the mounting block 12 in such a way that the side with a bulging diaphragm 26 faces the interior of the mounting block 12. When the diaphragm 26 vibrates, a high tone sound is produced. The other side of the acoustic diaphragm assembly 14, on the other hand, hides a circular magnetic field generator (not shown). When a proper electrical signal is fed into a magnetic coil inside the generator via wires 28, a rapidly fluctuating magnetic field is produced. The changing magnetic field in combination with the metal base 18 can induce vibration of the diaphragm 26.
The plastic pad 16 is a component for stationing the acoustic diaphragm assembly 14. Inside the metal base 18, there is a circular recess cavity 34 for accommodating the circular magnetic field generator on the acoustic diaphragm assembly 14. The metal base 18 has a plurality of holes 32 around the peripheral region for accommodating the screws 30 after it passes through the mounting block 12.
FIG. 2 is a diagram showing the components of the metal base illustrated in FIG. 1. The metal base 18 comprises a metal plate 35, a magnetic plate 36 and a base block 38. The metal plate 35 and the magnetic plate 36 are both circular and their diameters are the same. The metal base 18 has a circular recess cavity 37 in the middle with an internal diameter slightly larger than the diameters of the metal plate 35 and the magnetic plate 36. A circular recess cavity 34 (refer to FIG. 1) remains even after the magnetic disk 36 and the metal disk 35 are slipped into the recess cavity 37 in sequence. As mentioned before, the cavity 34 is used for housing the circular magnetic generator on the acoustic diaphragm assembly 14.
In addition, there is a stationing hole 39 in the center of the recess cavity 37 for fixing the magnetic plate 36 via the base block 38 with a guiding pin (not shown) from below. A few concentric grooves (not shown) of different sizes are also formed on the bottom surface of the base block 38. These concentric grooves are aids for dissipating the heat generated by the speaker. Without any mechanism for dissipating the heat, the magnetic plate 36 may rise to a temperature over 120.degree. C. resulting in its demagnetization. Since the base block 38 is formed in a single piece, exposed surfaces are quite limited. Hence, its heat dissipation capacity is intrinsically poor. Furthermore, due consideration must be made regarding the design of cavity 37 so that excessive magnetic flux leakage is prevented. This leakage occurs because some material on the opposite side of the cavity must be carved away to form the concentric grooves. Hence, the base block 38 is harder and more costly to build.